8
\$\begingroup\$

Any amplifier or A/V receiver that I have seen has a black and a red terminal for each speaker. From 2 pairs for a simple stereo system up to 7 or more for an A/V receiver.

It is quite a while since I have looked inside an amplifier but, for at least some, the blacks are common and connected to the chassis. Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?

Edit: Chassis might not have been the best word; as people seem to have picked up, the significance is whether they are all necessarily at the same potential. My outlook is still biased by early experience with valve devices: radios, TVs, amplifiers, in which a metal chassis (not an external casing) was usually a common 0V reference.

\$\endgroup\$
6
  • 5
    \$\begingroup\$ Not always. Bridge mode amplifiers drive both speaker connections from a different amplifier. They do this to get the maximum power from a limited voltage source - like the 12V battery in a car. \$\endgroup\$ Apr 26, 2020 at 14:02
  • 1
    \$\begingroup\$ @BrianDrummond different = differential? or rather differential output? \$\endgroup\$
    – DKNguyen
    Apr 26, 2020 at 19:54
  • 3
    \$\begingroup\$ @DKNguyen Different amplifier. Two ampliiers connected in anti-series (i.e. in anti-phase), with the speaker connected to both hot terminals. \$\endgroup\$
    – user207421
    Apr 27, 2020 at 4:35
  • 3
    \$\begingroup\$ Even if they ARE chassis, you most probably cannot take much of advantage of that. If you connect, for example, left and right speaker with common black, you will get pretty much audible cross-talk between left and right. Other "ground loops" are also possible. That's why the only meaning of these colors should be assumed to be polarity. \$\endgroup\$
    – fraxinus
    Apr 27, 2020 at 9:35
  • 1
    \$\begingroup\$ Assumptions are the cause of half the smoke I've seen. What triggered this question? \$\endgroup\$
    – Mast
    Apr 28, 2020 at 6:26

5 Answers 5

9
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Two common amplifier arrangements.

Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?

Bridge mode amplifiers use two amplifiers - one an inverted version of the other. The advantage is that on the same supply voltage you can now get double the peak to peak voltage. This is common in car audio systems where the supply voltage is fixed at 12 V nominal, but it is increasingly common in domestic amplifiers also.

The result is that you can't assume that black is common ground. Investigation is required.

\$\endgroup\$
5
  • \$\begingroup\$ With a bridged configuration, you get four times the power. \$\endgroup\$ Apr 26, 2020 at 22:36
  • 4
    \$\begingroup\$ Only if the amplifier can feed twice the current and the PSU can deliver twice the power. I deliberately avoided that as it wasn't relevant to the question. \$\endgroup\$
    – Transistor
    Apr 26, 2020 at 22:39
  • 2
    \$\begingroup\$ Having enough current usually isn’t the problem. In a car stereo it’s voltage clipping in the final stage with a 12V rail. The PSU is the battery and alternator. \$\endgroup\$ Apr 26, 2020 at 22:46
  • \$\begingroup\$ The other big advantage of bridge mode is that you can run off a single rail supply without needing large expensive output coupling capacitors. \$\endgroup\$ Apr 27, 2020 at 19:18
  • \$\begingroup\$ This answer, I am disappoint. The OP specifically asked about A/V receivers. Which are line powered. Which makes using bridge mode superfluous, given that even if you limit the swing to 50Vac SELV limit, that’s 300W into 8 ohms, single-ended. More than enough for a home theater setup with a powered subwoofer. Amplifiers are a different story. \$\endgroup\$ Apr 28, 2020 at 2:12
6
\$\begingroup\$

Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?

In many amplifier designs (especially the power-efficient and well-integrateable and hence popular class D driving a H-bridge), both speaker conductors might alternate between any voltages within the range of the internal power supply.

Especially in stereo applications that means that left and right "black" can't be at the same potential for every point in time.

\$\endgroup\$
5
\$\begingroup\$

I don't know if there is some sort of home-entertainment industry rule, but it certainly is very common practice. NOTE that the black terminal is the reference potential ("system ground" or "signal ground"), but not necessarily connected directly to the chassis. The system ground might be floating with respect to earth ground.

Separate from that, be wary of class D amplifiers or linear amplifiers with a BTL (bridge-tied load) topology. In these cases, the black terminal is not connected to the system ground - it has 1/2 of the output audio signal on it, and the signals are different for each channel, so the black terminals are not interchangeable. Red and black still indicate correct phasing.

\$\endgroup\$
3
\$\begingroup\$

For most home A/V gear, yes, the black is ground (chassis ground). You can check this with an ohmmeter.

Why? Typical A/V gear will use a line-derived high voltage supply with a half-bridge driver, capacitively coupled to the output.

Again, why? A line-powered amp has lots of voltage to work with. There is plenty of headroom to avoid voltage clipping, even at high power levels using just a single-ended drive. So it's just not worth the extra expense of using a full-bridge driver - even a switchmode one - for normal consumer A/V gear like a receiver.

Car stereos - not so much. Higher power in-dash ones will use a full-bridge drive to maximize the use of the 12V available.

Example: single-ended 12V power amp into 4 ohms:

  • Largest AC RMS output: 12V / 2 * 0.707 = 4.24V
  • Power into 4 ohms = 4.24^2 / 4 = 4.5W

Bridged 12V power amp into 4 ohms:

  • Largest AC RMS output: 12V / 2 * 0.707 = 4.24V
  • Bridged: 4.24V * 2 = 8.48V
  • Power into 4 ohms = 8.28^2 / 4 = 18W

That's a big win for a car stereo, so it's worth the extra expense.

Now, car power amps are a different matter. These will use DC-DC step-up make higher rails to achieve higher voltage swings at the output. Some may use bridge, some will not.

\$\endgroup\$
3
\$\begingroup\$

In many older vacuum tube-based radios with inductively-coupled output stages (not utilizing negative feedback), it was common to have the speaker connected directly to the secondary of the output transformer (T15 in this example) floating it completely.

enter image description here

This obviously wouldn't be used in a modern consumer-designed amplifier, but an example of where the speaker output wouldn't be tied to chassis or even circuit ground.

EDIT:

Inspired by @hacktastical's comment I looked up another schematic with multiple secondary taps. I don't have one for a McIntosh 240 so this is for a Fisher 500C, also a home/consumer integrated amplifier.

enter image description here

I expected this to be more straightforward, but looks like the 4 ohm tap is tied to circuit ground, and the "common" (I assume this is why historically it isn't called "ground") floated below circuit ground via an inductor in series (or I'm not familiar with the construction of a transformer symbol where a winding extends past the core). Either way, another example of where "common" or "ground" may not be circuit or chassis ground.

Fisher generally had (back then) good reasons for why they did what they did. Would be curious if anyone has any thoughts on the advantages of this design?

(Mod note: if this is veering off topic of the OP I can re-post as new. Now I'm genuinely curious.)

\$\endgroup\$
4
  • \$\begingroup\$ Some even had multiple impedance taps, like the McIntosh 240 I grew up listening to. It could do 4, 8 or 16 ohms, and could be bridged. \$\endgroup\$ Apr 28, 2020 at 2:16
  • \$\begingroup\$ @hacktastical your comment inspired me to look at designs of that era -- see my updated post. \$\endgroup\$
    – 640KB
    Apr 28, 2020 at 16:47
  • \$\begingroup\$ I found the McIntosh 240 here: ampslab.com/vintage_mcintosh_mc240.htm. It has a 'COM', 4, 8 and 16 ohm terminals on each side that are floating. \$\endgroup\$ Apr 28, 2020 at 16:54
  • \$\begingroup\$ Mods? Sidebars like these help keep SE fresh, even if sometimes it's just elder folks reminiscing... \$\endgroup\$ Apr 28, 2020 at 16:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.